1. Field of the Invention
(0001) The present invention relates to methods and apparatus for the nonlinear processing of delta-modulated pulse stream. The analog input signal is first converted into a one-bit high density pulse stream using the delta-sigma modulator (Δ-ΣM) as an analog-to-digital converter (ADC). Δ-ΣM is highly oversampled and its pulse stream is nonlinearly processed in the rectifying encoder (RE), then filtered and fed back into a reference input of the Δ-Σ modulator. This circuit is known as the root-mean-square to direct current converter (RMS-to-DC). Thus, the field of this invention is a nonlinear processing of Δ-ΣM one-bit pulse stream and it belongs to the area of non-linear digital signal processing (DSP).
2. Description of the Prior Art
There are a number of published references dealing with integrated RMS-to-DC converters. The majority of these publications deal with analog processing methods using trans-linear property of bipolar circuitry. These converters are difficult to implement in standard CMOS processes.
In the digital processing methods, which use high-speed n-bit ADC, the RMS value of an input signal is calculated by ordinary DSP calculation methods. The problem is that the digital computation unit occupies a large chip area and thus, does not lead to a low-cost solution.
To cope with analog component limitations, the use of Δ-Σ modulation is proposed in reference [1]. A similar approach is used by Linear Technology Inc. [2]. In both approaches an analog signal is Δ-Σ modulated and then multiplied (in a multiplying DAC) by a polarity signal at the output of Δ-Σ modulator. This mixed processed signal is then filtered and fed back to the negative input of the Δ-Σ modulator as a reference signal. When the reference input signal of a Δ-Σ modulator is taken into account, then the output of the modulator can be interpreted as a ratio-metric function plus quantization noise [U.S. Pat. No. 6,587,061 B2].
The mixed mode analog/digital multiplier, which performs the squaring operation of the analog signal by means of Δ-ΣM, is known as a polarity switch, and is proposed in reference [U.S. Pat. No. 6,285,306 B1].